464 MALL. [Vol. V. 



and ventral sides, 1 is much the same in transverse section, the 

 thickness gradually diminishing as the tail is approached. 



Cranial Nerves. 



The olfactory pit is sharply defined and is composed of five 

 or six layers of cells. Throughout its extent cell divisions are 

 present, most numerous, however, at its concave oral side. In 

 this region there are marked pyramidal cells with their base 

 towards the outside of the body and their apices pointing towards 

 the brain. These cells undoubtedly mark the beginning of the 

 olfactory nerve as pointed out by His. In Amblystoma and 

 Necturus these cells are much more pronounced and can be 

 traced in the various stages from the olfactory pit to the 

 brain. 



There is as yet no indication of a permanent optic nerve. 

 The primary optic vesicle still communicates with the fore-brain 

 while the distal or rod-and-cone layer of the secondary vesicle 

 shows peculiar changes. The layer is about twice as thick as 

 the proximal or pigment layer, and in both these are nuclear 

 figures. The location of these cells, which are dividing, is on the 

 margin which corresponds with the layer about the central 

 canal in the spinal cord. The pigment layer is about five cells 

 thick, while the rod-and-cone layer is about eight. 



The rod-and-cone layer is composed of two distinct zones, — a 

 distal or hyaline and a proximal or cellular. The hyaline zone 

 lies next to the lense and seems to be composed of cilia, all 

 being directed towards a centre lying within the lense. The 

 granular zone is composed in great part of round cells between 

 which are many bipolar and unipolar cells. The unipolar cells 

 are more numerous than the bipolar and project, with their pole, 

 towards the position which is later to be rods and cones. 

 The bipolar send one pole in the same direction and the other 

 into the hyaline layer. 



At a point in the stem of the optic vesicle nearest the mouth 

 there enters a vessel which is undoubtedly the arteria centralis 

 retinae. As this perforates the hyaline layer, the "cilia" be- 

 come shorter, but are in no way directed towards this artery, 



1 His, Deckplatte, Bodenplatte. 



